Radiation cure of ethylenically unsaturated coating compositions is well known. Ultraviolet radiation is particularly desirable for this purpose, but it does not penetrate opaque pigments very well. When the coating is to be extensively pigmented, as in coatings which are pigmented with magnetizable oxide particles to form magnetic recording structures, more penetrating ionizing radiation is desired. Electron beam radiation provides the preferred curing radiation.
To provide good film properties, the unsaturated materials present in the coating composition include polyethylenically unsaturated materials, which are preferably oligomers. These polyethylenic materials, and especially those which are oligomers, possess poor wetting characteristics, so the grinding of the pigment into the polyethylenically unsaturated vehicle becomes a significant problem. If the pigment is not properly wetted and dispersed in the vehicle, it will not be uniformly dispersed in the cured film.
Some progress has been made in providing polyethylenically unsaturated oligomers which possess improved wetting characteristics, but it is desired to maximize the pigment wetting capacity since this minimizes the difficulty of dispersing the pigment into the polyethylenically unsaturated vehicle. The grinding operation is time consuming and costly.